Method of gear changing in a motor vehicle

ABSTRACT

At the time of automatic gear changing without declutching in a mechanical stepped gearbox ( 6 ) in a motor vehicle, the speed of the gearbox output shaft ( 36 ) is detected and any speed change occurring immediately after disengagement of a gear to neutral position results in adjustment of a predetermined zero-torque level for the engine as a function of the speed change.

TECHNICAL DATA

The invention relates to a method of automatic gear changing in amechanical stepped gearbox, and particularly to adjustment of the torquedelivered by the engine during a gear change.

STATE OF THE ART

Automatic gear changing in mechanical stepped gearboxes requires thetorque delivered from the engine to be adjusted in order to reduce thetorque transmitted at the point of contact of the relevant gears. At thetime of gear disengagement it is desirable for there to be a torque-freestate at the contact point between the gears in the gearbox, but sinceit is not practically possible to measure the torque at the gear contactpoint the torque adjustment has to be achieved indirectly by adjustingthe torque delivered from the engine. SE,C,9401653-2 refers to asolution whereby gear changing takes place without the disc clutch beingreleased. In this case a gear change is preceded by the engine torquebeing adjusted towards a zero-torque level which is intended to lightenthe load on the gearbox during the gear change. At this stage the torquewith which the engine is modulated is calculated on the basis ofavailable data concerning the engine, its moment of inertia, itsinternal torque and the like. The modulated torque may also depend onwhether any power take-offs are operating or not.

Disadvantages of this method are that there is no direct measurement ofthe torque and that the modulation is based on the expected behavior ofthe engine. It is therefore not known whether the modulated torque iscorrect or not. The manufacture of engines and the various drivelinecomponents may involve tolerance differences which result in eachindividual vehicle having different characteristics. Even if thebehavior of an engine can be predicted correctly when it is new, itscharacteristics will change over time and with engine wear. This meansthat the engine torque modulated over time is probably not the same aswhen the engine was new. This in its turn means that gear changing maybecome difficult and/or that it is unacceptably delayed in certaincircumstances. Oscillations in the control system may result in a verylong scaling-in time being required for being able to determinecorrectly the fuel quantity which gives constant synchronous speed.

SE,C,9600454-4 refers to a solution which aims at improving thatdescribed above. To this end, a measurement is carried out at each gearchange to ascertain whether the modulated zero-torque was correct. If itwas not, the incorrect torque detected is used to correct the originalzero-torque value so that before a subsequent gear disengagement theengine is modulated with a more correct zero-torque value. During themeasurement, the amount of change in the engine speed with the fuelsupply unchanged is registered for a certain measuring period after thegear disengagement and with the gearbox in neutral position. Knowing thelength of the measuring period and the engine's moment of inertiaprovides a measure of the corresponding torque deviation, i.e. theamount of torque by which the zero-torque value applied needs adjustingso that gear disengagement can take place in a torque-free state of thegearbox. The corrected torque level is used as a basis for a calculationwhich determines when correct gear disengagement will take place.

A disadvantage of this method is that the measurement takes a relativelylong time, approximately 0.5 second or longer, thereby adverselyaffecting total gearchange time. Particularly when driving on steepuphill runs with heavy loads it is desirable to have the shortestpossible gearchange time so that the vehicle will not lose too muchvelocity during gear changes.

Experience also shows that for technical reasons of engine design it hasproved difficult in the control of new engine systems to achieve asimple match between the fuel quantity supplied and the torque obtainedat the low loads which prevail at the time of gear changes. A givenquantity of fuel does not always result in the intended engine torque,so setting the desired zero torque becomes unreliable, with consequentlyproblematic gear changing.

Zero torque not prevailing at the time of gear disengagement isprimarily perceived as a disturbance to comfort whereby an oscillationis initiated in the driveline. This oscillation then causes a time delaybefore the next gear can be engaged. The engine's zero torque dependsmainly on its internal friction and may therefore vary with such factorsas wear, temperature and engine speed. The determination and correctionof the zero torque has therefore to be carried out at regular intervalsso that there is an applicable value of it for the whole life of thevehicle.

OBJECTS OF THE INVENTION

One object of the invention is to make it possible more reliably thanpreviously to ensure that the torque transmitted in the gearbox will benil at the time of gear changes. A further object is to be able toshorten the time between disengaging one gear and engaging the next.

DESCRIPTION OF THE INVENTION

The objects indicated above are achieved by the invention having aneffective engine torque level adjustment for affecting the gear change.A control system reduces the torque transmitted in the gear box at thetime of the gear change by adjusting the torque delivered by the engineto a predetermined zero torque level. Disengagement of a gear to theneutral position takes place at that zero torque level. The zero torquelevel is corrected between gear changes if there is a difference betweenthe predetermined zero torque level and the actual zero torque levelthat is sensed. For this purpose, the speed of the gear box output shaftis detected. Then there is a speed change during a predetermined shortperiod of time immediately after disengagement of the operative gear tothe neutral position which adjusts the predetermined zero torque levelas a function of the speed change. In particular, a speed change in theform of an increase raises the zero torque level while a speed change inthe form of a speed reduction lowers the zero torque level. Thepredetermined short period of time amounts to not more than 50 ms andthe adaptation of the engine speed for engaging another gear, inparticular, a different gear commences at the time of disengagement ofthe previous gear. The solution adopted according to the inventionresults in the engine torque adjustment being possible with greaterprecision than previously. It also makes it possible to shorten the timeneeded between disengaging one gear and engaging the next.

The method according to the invention makes it possible to achieve morereliable and quicker gear changes, particularly when driving with heavyloads on uphill runs.

Other features distinguishing the invention are indicated by thefollowing description of an embodiment with reference to the attacheddrawings.

LIST OF DRAWINGS

FIG. 1 shows a schematic structure of a gearchange system for mechanicalstepped gearboxes in a vehicle, and

FIGS. 2 and 3 show schematically the speed of the gearbox output shaftas a function of time during gear disengagement when the engine torqueis respectively too low and too high.

DESCRIPTION OF AN EMBODIMENT

FIG. 1 shows a gearchange system for monitoring and controllingcomputer-assisted gear changing in a mechanical gearbox in a motorvehicle. The structure and operation of the system correspond largely tothose of the system described in the aforesaid SE,C,9401653-2 andSE,C,96004544, so the only components depicted are those required forunderstanding the present invention.

The vehicle is powered by a combustion engine 2, preferably a dieselengine, which is connected via a clutch 4, a mechanical stepped gearbox6, a propeller shaft 8, a final gear and a driving shaft 12 to thevehicle's driving wheels 14. In this embodiment the clutch 4 is onlyintended to be operated manually at low vehicle speeds and when startingand stopping. At the time of gear changes while the vehicle is otherwisein motion, the clutch 4 is intended to be operated neither manually norautomatically, so gear changes then take place with the clutch 4 engagedand acting as a drive power transmitting connection between the engine 2and the gearbox 6.

This gearchange system effects gear changes partly by adjusting theengine speed and the engine torque at the time of gear changes andpartly by operating the servos in the gearbox so as to disengage theoperative gear and engage the next gear.

The clutch 4 remaining engaged during gear changes results in severeengine control requirements if gear chances are to take place with theshortest possible torque break in the mechanical stepped gearbox andwith torque-free disengagement of a gear and good speed synchronizationat the time of engagement of the next gear.

The gearchange system incorporates a control unit 16 provided with amicrocomputer and connected by various lines to various components ofthe system. These connections are used for transmitting various signalsas illustrated by arrows in FIG. 1. The control unit 16 also has two-wayconnections whereby it sends output signals to and receives inputsignals from a multiplicity of control units via a number of linksrepresented in FIG. 1 by bidirectional arrows. Via a link 18 with theengine's fuel injection system 20, which is itself connected to varioussensors (not further described here), the control unit 16 receivesinformation on the engine speed. The control unit 16 also receives viathe link 18 corresponding information on the engine torque, which is inpractice determined on the basis of the fuel quantity injected. Via alink 22 the fuel injection system 20 adjusts the fuel quantity suppliedto the engine's injectors. During gear changing the control unit 16provides the fuel injection system 20 with signals which cause theengine 2 to be modulated with the desired torque.

Via a line 24 the control unit 16 receives from a temperature sensor inthe engine a signal representing the engine temperature or, in practice,the temperature of the engine coolant. A sensor 26 provides the controlunit 16 via a line 28 with information on whether any power take-offsare operating and imposing extra loads on the engine.

Via a link 30 the control unit 16 is connected to various solenoidvalves in the gearbox 6 which activate the servos to engage anddisengage gears. This link 30 is also used for providing the controlunit 16 with signals which represent the operating state of the gearbox6 and indicate which gear is engaged and, during gear changing, withgearchange signals which represent the various stages of the gearchange.

Via a line 32 the control unit 16 receives from a speed sensor 34 on thegearbox output shaft 36 information on the latter's speed. The controlunit 16 is also connected to other vehicle operating controls (notdepicted), e.g. gear selector, accelerator pedal, brake pedal, retarder,etc. As these controls operate conventionally and do not distinguish themethod according to the invention, further description of them isomitted.

Gear changing is initiated entirely automatically if the driver hasselected an automatic position or manually if the driver has selected amanual position. Irrespective of whether gear changing is initiatedautomatically by the control system or manually by the driver, it iseffected by the control unit 16 without the clutch 4 having to beoperated.

For a gear change to take place smoothly it is generally desirable firstto reduce the engine torque to a level corresponding to zero torque inthe gearbox, so that neutral position can be reached. To change to a newgear, the engine speed is thereafter adjusted so that it matches thepropeller shaft speed, i.e. the speed of the gearbox output shaft.Thereafter the engine torque is increased to a level desired by thedriver.

A gear change initiated in a wrong torque situation, e.g. because thepredetermined zero-torque level was not correct, is primarily perceivedas a disturbance of comfort whereby an oscillation occurs in thedriveline. This oscillation results in longer gear changing time, sincethe next gear cannot be engaged until the oscillations fade away. Italso results in the vehicle being jerked, which is disturbing for thedriver and causes unnecessary wear on mechanical components, e.g. thegearbox.

With the type of gear changing to which the invention refers, at thecommencement of a gear change the engine torque is reduced according toa predetermined torque profile. Gear disengagement to neutral positionis then initiated at a time selected such that the actual geardisengagement will take place when the torque transmitted in the gearboxis nil. It is well known that this zero-torque level will to some extentvary as a result of varying operating conditions, wear, etc.

It has according to the invention been found possible by studying thespeed of the gearbox output shaft 36 at the time of gear disengagementto gain a good understanding of how the zero-torque level used needs tobe adjusted.

The invention is based on the fact that when the vehicle is runningthere will be a certain twist of the drive shaft due to the transmittedtorque. The propeller shaft will act like a pre-stressed spring. Duringa gear change this spring/shaft will be “released” and there will be achange in the detected speed. This detected speed change will have acertain amplitude corresponding to how much the transmitted torquediffers from the predetermined zero torque level, and it will have adirection (increase or decrease) corresponding to if the predeterminedzero level was too low or too high. This is shown on FIGS. 2 and 3.

FIG.2 depicts a situation in which disengagement of a gear to neutralposition at time to takes place when the when the shaft has beenprestressed too much, or; when the engine's zero torque is too low. Theresult is that the detected speed n of the gearbox output shaft 36initially increases, i.e. assumes a positive amplitude, before itapproaches the value which corresponds to the vehicle speed. Thereforethe zero torque level should be raised. The time Δt for the firstoscillation between t₀ and t₁ on the curve is very short, of the orderof approximately 20 ms. Studying the change in the gearbox output shaftspeed therefore requires its measurement with great accuracy at the timeof gear disengagement.

FIG.3 shows a contrasting situation in which disengagement of a gear toneutral position at time t₀ takes place when the shaft has beenprestressed insufficiently, or; when the engine's zero torque is toohigh. The result in this case is that the detected speed n of thegearbox output shaft 36 initially decreases, i.e. assumes a negativeamplitude, before it approaches the value which corresponds to thevehicle speed. Therefore the zero torque level should be lowered. Thetime Δt for the first oscillation between t₀ and t₁ on the curve is ofthe same order of magnitude (approximately 20 ms) as in the previouscase.

It thus emerges that ascertaining whether the speed change initially hasa positive or negative amplitude also makes it possible to ascertainwhether the engine zero torque applied at the time of a gear change wastoo low or too high. This provides a basis for increasing or decreasingthe zero torque for the next gear change, i.e. for updating the zerotorque before a subsequent gear change.

It has in practice been found that necessary information on theappearance of this speed curve can be obtained with a measuring timewhich need not be longer than approximately 50 ms, since only the firstoscillation on the curve is really of interest and experience shows thatit is of sufficiently short duration to be comprised within thatinterval.

The magnitude of the speed amplitude provides some idea of the magnitudeof the zero torque error and can for small amplitudes and small torqueerrors provide a basis for the required torque correction.

A method which has proved simpler in practice is to apply, when a speedchange is observed, a predetermined torque correction in the desireddirection and thus at each successive gear change to allow the controlsystem to seek out a zero-torque value at which the speed change isacceptably small. After a relatively small number of gear changes thecontrol system normally achieves the desired updating, which isnaturally executed for a value matrix pertaining to the particularoperating situation, since different operating situations, e.g. with orwithout the operation of certain auxiliary devices which impose loads onthe engine, result in different engine operating situations.

An important advantage of the method according to the invention is thatit makes it possible to ascertain very quickly after a geardisengagement whether and how the zero torque needs updating. This meansthat in practice it is possible immediately after gear disengagement tobegin adjusting the engine speed to a suitable value for the next gear.This is particularly useful when driving with heavy loads on uphillruns, since gear changes can take place more quickly and the vehiclewill not lose so much velocity during gear changes.

In other words, it will no longer be necessary to delay gear changing inorder to be able, for a certain period of time during gear changes, tostudy how the engine behaves at a supposed zero-torque load with thegearbox in neutral position.

A further advantage is being able to obtain a reliable indication of howthe zero-torque load will change without having to keep the engine's sfuel supply at such a low level that the zero-torque level is maintainedlong enough for carrying out desired measurements on the engine. Thishas in particular been found in practice to entail considerabledifficulties in achieving sufficient accuracy in modulated zero-torque,inter alia because of hydraulic phenomena in the fuel system. Suchdifficulties are avoided according to the invention by measurement at along-lasting zero torque no longer being necessary.

The solution adopted makes reliable and smooth gear changing possiblewith a simple mechanical gearbox which is considerably less expensivethan an automatic gearbox and is also more efficient.

What is claimed is:
 1. A method of automatically changing gears in amechanical step gearbox in a motor vehicle, wherein the gearbox has anoutput shaft that operates at a measurable speed; the method comprisingthe steps of: reducing torque transmitted in the gearbox at the timegears are to be changed by adjusting the torque delivered by an engineto a predetermined zero torque level, then disengaging a then operativegear to place the gearbox in a neutral position while retaining a clutchbetween the engine and the gearbox in engagement; later engaging a gearfor completing a gear change; and correcting the predetermined zerotorque level between gear changes if there is a difference between thepredetermined zero torque level and the actual zero torque level, thecorrecting comprising detecting a speed of a gearbox output shaft duringa predetermined short period of time immediately after disengagement ofthe operative gear to the neutral position and adjusting thepredetermined zero torque level toward the actual zero torque level as afunction of a speed change detected during the short period of time. 2.The method of claim 1, wherein the predetermined zero torque level isincreased if the detected speed change is a speed increase.
 3. Themethod of claim 2, wherein when the predetermined zero torque level isdecreased if the detected speed change is a speed reduction.
 4. Themethod of claim 1, wherein when the predetermined zero torque level isincreased if the detected speed change is a speed reduction.
 5. Themethod of claim 1, wherein the predetermined period of time is not morethan 50 ms.
 6. The method of claim 1, wherein correction of the zerotorque level is made in proportion to the amplitude of the detectedspeed change of the gearbox output shaft.
 7. The method of claim 1,further comprising commencing of adaptation of the engine speed forreengaging of a different gear at the time of disengagement of a gear.8. The method of claim 2, wherein the gear that is reengaged is adifferent gear than the operative gear previously disengaged.